1. Field of the Invention
The present invention relates to an image display system, or more particularly, to an image display system for displaying a raster image such as outline fonts or picture data while superimposing it on part of a frame image.
2. Description of the Related Art
In image display systems, while the picture area is scanned in order to display a frame image, a raster image such as outline fonts or picture data must be transferred from a memory, in which the raster image is stored in advance, within a predetermined period of time. The predetermined period of time is determined based on a frame frequency or a resolution. If the transfer of the raster image is not completed within the predetermined period of time, the raster image is not correctly superimposed on a frame image. Therefore, the time required for transfer of the raster image must be shortened.
An image display system disclosed in Japanese Unexamined Patent Publication No. H10(1998)-161638 includes a font data conversion circuit 122 and a font data address production circuit 123. A video memory 136 includes a video memory plane 103. In response to an output of a scanning line count means that indicates a scanning line in which characters are contained for display, the video memory plane 103 is accessed via a memory interface 124 in order to read character text codes, which are contained in respective scanning lines, sequentially from successive spaces. Font data that is display data is then transferred, and an image signal is finally transmitted from a display circuit 125 to a cathode-ray tube (CRT).
Consequently, even if characters to be displayed while being contained in the same scanning line are represented by character codes whose locations are separated from one another, since font data is stored in a video memory area of 256 bytes long in units of pixels to be contained in a scanning line, the font data can be transferred owing to the feature of a DRAM permitting fast access. Eventually, the time required for transfer can be shortened.